The instant invention relates to chemical analysis methods for analyzing polyethylene glycol compounds. More particularly, the instant invention relates to the analysis of polyethylene glycol compounds by liquid chromatography under “critical conditions”. The polyethylene glycol compounds of the instant invention have been “activated” to facilitate chemical modification of physiologically active materials, which modified materials are applicable, for example, in drug delivery systems.
Biologically active compounds conjugated with polyoxyalkylenes can provide enhanced biocompatibility for the compound, See, for example, U.S. Pat. No. 5,366,735 and U.S. Pat. No. 6,280,745. A review of this subject by Zalipsky, in Bioconjugate Chem., 1995, 6, p 150-165, identified polyethylene glycol as one of the best biocompatible polymers to conjugate with a biologically active compound (such as a drug, a protein, a peptide or an enzyme) to produce a conjugate having improved properties such as compatible solubility characteristics, reduced toxicity, improved surface compatibility, increased circulation time and reduced immunogenicity.
Polyethylene glycol (PEG) is a linear polyoxyalkylene terminated at the ends thereof with hydroxyl groups and generally represented by the formula: HO(CH2CH2O)nH. Monomethoxy polyethylene glycol (mPEG) is generally represented by the formula: CH3O(CH2CH2O)nH. mPEG can be “activated” with a group “A” that will couple with a group of the biologically active material. Activated mPEG is generally represented by the formula: CH3O(CH2CH2O)nA. For example, trichloro-s-triazine activated mPEG will couple to an amine group of a biologically active material, as discussed by Henmanson in Chapter 15 of Bioconjugate Techniques (1996).
More recently, so called “second generation” PEGylation chemistry has been developed to, for example, minimize problems of diol impurity contamination of mPEG, to increase the molecular weight of the mPEG and to increase stability of the conjugate, see Roberts et al., Advanced Drug Delivery Reviews 54 (2002) p 459-4. U.S. Pat. No. 6,455,639 described an increased molecular weight mPEG having narrow molecular weight distribution.
Liquid chromatography under critical conditions has become an important method for polymer analysis, see, for example, Gorbunov et al., J. Chrom A, 955 (2002) 9-17. Liquid chromatography under critical conditions has been used to determine polyethylene glycol in mPEG (see, for example, Baran et al., J. Chrom. B, 753 (2001) 139-149; and Kazanskii et al., Polymer Science, Series A, Vol 42, No. 6 (2000), p 585-595. However, the degree of resolution of the polyethylene glycol and mPEG peaks is poor when the molecular weight of the mPEG is 5,000 grams per mole or more (see FIG. 2 of the Kazanskii et al. reference). And, liquid chromatography under critical conditions has not been used to analyze activated mPEG.